The significance of Tournaisian tectonism in the Dublin basin: Implications for basin evolution and zinc-lead mineralization in the Irish Midlands
Introduction
Sediment-hosted ore deposit studies often concentrate on understanding the sulphide mineralization and geology in the immediate vicinity of the mineral occurrence (Ashton et al., 1986, Jones and Brand, 1986). However, the large-scale nature of most sedimentary fluid flow systems that produce ore deposits would suggest that it is necessary to understand not only the nature of mineralization but also the nature of the basin in which it is hosted. The Irish Carboniferous succession is a world-class mineral field where the study of the host basin is a prerequisite for understanding the nature of mineralization. Zn-Pb deposits in the Irish Midlands are characterised by strong structural and stratigraphic controls, with sulphide occurrence concentrated in the hangingwall of large normal faults (Ashton, 1995, Hitzman, 1995, Johnston et al., 1996, Leach et al., 2005). The origin of these “Irish-type” deposits is still much debated, with genetic scenarios ranging from synsedimentary exhalative (e.g. Boyce et al., 1983, Banks, 1986) through to a burial origin (Shearley et al., 1996, Peace and Wallace, 2000, Reed and Wallace, 2001, Reed and Wallace, 2004, Hnatyshin et al., 2015).
The Irish Midlands is a classic example of an onshore sedimentary basin where there has been a long history of stratigraphic and sedimentological research and yet a clear understanding of the large-scale basin evolution remains elusive. This is probably due to the general lack of outcrop in the Irish Carboniferous, which has led to a dependence on diamond drill cores to delineate the geology. In turn, problems in determining the lateral continuity of stratigraphic units have become apparent and by necessity, Irish stratigraphy (and stratigraphic nomenclature) is dominated by studies of relatively small areal extent.
In this study, we have examined the structural and stratigraphic setting of a region in central Ireland (Fig. 1) using recently acquired seismic profiles and wireline log data and integrating this with diamond drill cores. This approach has enabled an understanding of the basin history which had not been previously possible.
The key to this greater understanding has come from seismic profiles, which have provided a basin-scale view of the succession (100s of km laterally and several km deep). Gamma-ray logs have also provided an objective measure of lithology and in combination with lithological observations from the core, have enabled a deeper understanding of the sedimentology and stratigraphy of this well-studied basin. Moreover, an understanding of the basin history has provided insights into the origin of Irish-type zinc-lead deposits not previously available. We therefore suggest that using this integrated approach (combining the techniques used in subsurface petroleum exploration with those used in outcrop mapping and mineral exploration) may have wide application in other onshore basins globally.
Section snippets
Geological setting
The Lower Carboniferous stratigraphy of the Irish Midlands unconformably overlies faulted and folded Lower Palaeozoic rocks. (Phillips, 2001, Chew and Stillman, 2009). Uplift and erosion of the Lower Palaeozoic stratigraphy resulted from the Caledonian Orogeny (Phillips, 2001, Chew and Stillman, 2009). During the Upper Devonian, crustal extension and subsidence occurred in Ireland. This resulted in a progressive northward transgression, which facilitated deposition of alluvial sediments and red
Methods
The core described in this study was provided by Teck Ireland and in total, 18 holes were logged. The seismic survey was conducted by Teck Ireland in 2010–2011 and covered an area of approximately 725 km2. However, for the purposes of this study, a region of approximately 145 km2 around the Ballinalack and Keel area was considered (Fig. 3). Open-hole wireline logs were run on seven drill holes using a density gamma probe (trisonde). Natural radioactivity was measured in API units, at a step rate
Stratigraphy
Seismic stratigraphic interpretation reveals several distinctive units that can be directly correlated with lithological units recognised in core (Fig. 4, Fig. 5, Fig. 6). In addition, these units can be characterised by their gamma ray profile (Fig. 7).
Previous interpretations of carboniferous tectonism and mineralization
At Ballinalack, there is strong evidence that mineralization-related structures (Ballinalack High and associated faults) were formed during the Middle to Upper Tournaisian. This, together with evidence from across the Northern Midlands (Jones et al., 1988, Andrew, 1992, Pickard et al., 1992), suggests that the Moathill tectonic event was significant in Ireland and may have produced many of the normal faults associated with the Irish Zn-Pb field (Fig. 11). Despite this, most literature on the
Conclusions
Analysis of seismic and well log data from the onshore Irish Carboniferous succession indicate that the Tournaisian–Viséan section is dominated by two major tectonic events. These tectonic events each produced large amounts of basin subsidence that overwhelmed small-scale eustatic effects, causing two basin-wide transgressive events.
Seismic profiles and drillholes at Ballinalack indicate faults that are now mineralized as well as the Ballinalack High, which were produced during the Tournaisian
Acknowledgements
Many thanks are extended to all the people at Teck Ireland Ltd. for providing support for this research. Thanks also go to the Geology Department and its affiliates at Trinity College, Dublin, for providing valuable insight into the study area.
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